Use gpu::Mailbox instead of std:string in IPCs

gpu/command_buffer/common/mailbox.h

Index: gpu/command_buffer/common/mailbox.h
diff --git a/gpu/command_buffer/common/mailbox.h b/gpu/command_buffer/common/mailbox.h
index 938167f5cf8142b078cd32af506f1d10b4d384f9..d9d25d418d3eeb0c6843809c98092f76bf39ecee 100644
--- a/gpu/command_buffer/common/mailbox.h
+++ b/gpu/command_buffer/common/mailbox.h
@@ -21,6 +21,12 @@ struct GPU_EXPORT Mailbox {
   bool operator<(const Mailbox& other) const {
     return memcmp(this, &other, sizeof other) < 0;
   }
+  bool operator==(const Mailbox& other) const {
+    return memcmp(this, &other, sizeof other) == 0;
+  }
+  bool operator!=(const Mailbox& other) const {
+    return !operator==(other);
+  }
 };
 
 }  // namespace gpu